I think assuming that Bitcoin will be (for practical purposes) eternally deflationary is the safest bet that could be made today. The world economy has been growing by triple digits every century. Not even now we're nowhere near a global Star Trek post-scarcity future. So it's misleading to say that Bitcoin is deflationary "based only on a presumption". Somewhat like saying that physics is just "based on the presumption of Newton's laws".
Adding extra zeroes after the decimal point for money already in circulation leaves the total supply the 'same' but could also be viewed as increasing supply 10-fold, without disadvantaging anyone currently holding the currency.
Its never happened before, so I'm not sure many economists have thought about it
What inflation does in an economic sense is much bigger - it changes who has relatively more or less wealth or debt over time. If you take out a loan for $10,000 and inflation makes both your prices and wages go up 2x, then you now have half as much debt.
Deflation does the opposite and makes debt more and more expensive. This is why most economists are against it - debts that grow more expensive will trap people. Historically, debts were forgiven by decree (a jubilee) because the system kept breaking.
If Bitcoin was a predominant part of an economy borrowing a loan in it could be very bad. In a predominantly Bitcoin based economy everyone could have abundant wealth and there would be no need to be a debt slave.
My understanding is that regardless of how inflationary or deflationary Bitcoin is, there will be no effect on the availability for anyone to buy and sell.
what about Bitcoins being lost? perhaps we should also consider coins that are not used in the past five years also "lost".
At some point, the rate of loss may overtake the rate of new coin creation.
> perhaps we should also consider coins that are not used in the past five years also "lost".
Maybe, but be prepared for some of those to someday be spent, they may merely have been hoarded. I have dollars that have been in my possession for more than five years, they aren't lost as far as I am concerned.
Interestingly, though, while (some) seigniorage is a good thing for a debt-based like the dollar, it's a bad thing for Bitcoin.